The invention relates to a focussing device comprising a Luneberg lens and its manufacturing process. It relates more particularly to a focussing device comprising a Luneberg lens comprising a homogeneous volume of dielectric.
It is known, from French patent applications No. 98/05111 and No. 98/05112 filed on Apr. 23, 1998 in the name of the applicant to use a Luneberg lens in satellite-signal receivers, especially for the tracking of nongeostationary satellites.
In theory, the lens must be composed of a given number of layers of dielectrics high enough to approach the ideal model of refractive index variation characteristic of the Luneberg lens. The refractive index n relating to a layer and the relative dielectric constant E (or permittivity) corresponding to it are therefore related by the equation: n=Exc2xd. However, increasing the number of layers is limited in practice by strict manufacturing tolerances which are incompatible with a mass production process. For small-sized lenses, typically having a diameter of less than 40 cm for transmissions in the Ku band, one solution to this problem is to opt for a lens having a single layer of homogeneous dielectric.
For the purpose of reducing the overall size of the lens, it is therefore necessary to increase the density, with the disadvantageous consequence of increasing the weight of the lens. There must therefore necessarily be a compromise between the size of the lens and its weight. These volume and weight constraints impose a well-defined density range on the dielectric. For example, for a lens 35 cm in diameter, the allowed density is typically between 0.3 g/cm3 to 0.8 g/cm3.
It is known, from the prior art, to use, as dielectric, a compound comprising expanded polystyrene filled with high-density granules, ceramic or metal granules for example, in order to increase its density and to shift it toward the desired density range.
However, this type of compound does not allow there to be complete homogeneity of the granules in the compound, and therefore does not guarantee a homogeneous density within the volume of the lens. In addition, the compound obtained is expensive.
It is an object of the invention to remedy these drawbacks.
For this purpose, the subject of the invention is a focussing device comprising a Luneberg lens comprising a homogeneous volume of dielectric, characterized in that the dielectric comprises a granular agglomerate defined by a homogeneous granule size distribution of thermoplastic granules, at least one plurality of these granules being welded together by granule boundaries in order to keep said volume consolidated.
Thus, since the granules are placed one with respect to another in such a way that each of them is in contact with at least one other, the existence of solid granule boundaries makes it possible to fulfill the function of a binder between the various granules and to generate a compact assembly of granules.
According to one embodiment, said plurality of granules is included at least within an outer layer of said homogeneous volume, said outer layer being relative to the outer surface of the volume extended toward the inside of said volume to a predetermined depth, preferably of the order of a multiple of a received and/or transmitted half-wavelength. Consequently, the outer layer serves to keep the granules of material under pressure within the outer layer. In addition, the thickness of the outer layer defined as a multiple of a received and/or transmitted half-wavelength optimizes, from the electromagnetic standpoint, the exchange of signals with the outside of the lens and at the same time allows this lens to act as a radome.
Preferably, said plurality of granules is uniformly distributed within said volume. Thus, it is possible to ensure density homogeneity over the entire volume.
Advantageously, the permittivity xcex5r of the lens of material composed of thermoplastic granules with a permittivity xcex5r0 is connected with a fill factor F, denoting the ratio of the volume actually occupied by the granules to the total volume of said volume, by the equation:
xcex5r=[(1+2F) xcex5r0+2(1xe2x88x92F)]/[(1xe2x88x92F) xcex4r0+2+F]. 
French patent applications No. 98/05111 and No. 98/05112 describe in particular primary sources close to the focussing surface of the lens. According to one embodiment, the focal length of the lens depends on the refractive index n of the lens and on the accepted phase variation over the aperture of the radiation pattern of the lens, the refractive index of the lens being given by:
n=xcex5rxc2xd
In order to allow the radius of the lens to be increased and thus to decrease the focal length of the latter without increasing the phase variation over the aperture of the radiation pattern of the lens, at least one additional layer covers said homogeneous volume of dielectric, said additional layer also comprising an agglomerate of thermoplastic granules of a material differing from that of the granules of said volume and having a density less than that of said volume. In this way, by creating a gradation in the refractive indices from the outside of the lens toward the inside of the latter, it is possible to approach the ideal model of a Luneberg lens.
According to one embodiment, the granules of said volume are composed of polystyrene. Thus, the density of the lens is within the desired density range.
According to one embodiment, the granules of the second layer are composed of polypropylene.
In addition, the processes for converting thermoplastics (injection molding, thermoforming, rotomolding and compression molding) do not allow parts to be produced with a thickness greater than about fifteen millimeters. Furthermore, these processes entail density variations within the parts and, because of the phenomenon of material shrinkage, deformations and geometrical variations appear within the parts. These problems may, in particular, interfere with the proper operation of a lens as described above and implemented according to said processes.
It is also an object of the invention to solve these drawbacks and, more particularly, to provide a process for manufacturing a Luneberg lens comprising a homogeneous volume of dielectric.
For this purpose, the subject of the invention is a process for manufacturing a Luneberg lens comprising a homogeneous volume of dielectric, comprising a step of forming said volume, characterized in that the volume comprises a granular agglomerate defined by a homogeneous granule size distribution of thermoplastic granules and in that said process comprises the following steps:
a step of heating the volume in order to raise the temperature of at least one outer layer of the volume to a transition temperature between the softening temperature of said material and the melting point of the material, the outer layer representing the outer layer of the volume extended into the volume to a predetermined depth and the transition temperature being defined by a phase change toward a viscous phase of at least part of said outer layer over said depth;
a step of cooling said outer layer in order to harden said outer layer.
Thus, this hardened outer layer allows the material within the layer to be kept under pressure. The fact of not completely melting the thermoplastic allows the initial density of the thermoplastic to be maintained. Furthermore, using a thermoplastic is inexpensive.
Advantageously, during the heating step, the temperature is raised in order to melt at least the outer layer of the granules contained in said outer layer of the volume for the purpose of forming viscous granule boundaries binding the granules of the outer layer of the volume. Thus, the granules of the outer layer, arranged against one another and leaving room for voids forming an open porosity, are consolidated by the solidification, during the cooling, of the viscous granule boundaries encapsulating said granules. The fact of not entirely melting the granules of the outer layer makes it possible to allow air to flow sufficiently between them for rapid cooling. By virtue of this consolidation, the outer layer is converted into a shell keeping the granules of the volume under pressure.
According to one embodiment, the thickness of the outer layer is of the order of a multiple of a received and/or transmitted half-wavelength. This layer must have a thickness greater than a first value in order to be able to keep sufficient pressure on the mass of material that it contains.
Advantageously, the heating step is carried out until the entire volume has reached the transition temperature, the diffusion of the heat throughout the entire volume having the function of expanding all the granules. This expansion of the granules creates, within the volume, a pressure between the granules allowing the welding between the granules to be strengthened. Density homogeneity is provided throughout the volume.
According to one embodiment, the heating is carried out by convection. For example, the heating is carried out by blowing hot air.
To meet such thickness constraints, the heating time must be greater than a first temperature value in order to allow the outer layer of the granules of at least the outer layer of the volume to melt and must be less than a second temperature value in order to allow homogeneity of the volume as explained above.
According to another embodiment, the heating is carried out by radiation, for example ultrasonic radiation.
According to one embodiment, the process employs molding means for forming said volume, said molding means comprising porosities uniformly arranged over the outer surface of said molding means and the relative rotational speed of said molding means with respect to the blowing direction being less than a given speed in order to allow temperature homogeneity within at least the outer layer.
According to one embodiment, the molding means are vibrated in order to mix the material. Thus, better homogeneity is achieved.
According to one embodiment, the heating temperature and the heating and cooling times are adjusted according to the thermoplastic used and to the volume of thermoplastic to be obtained.
According to one embodiment, the process employs pressing means, the pressure of which depends on the desired material density in the volume.
According to one embodiment, at least one sheet of thermoplastic is thermoformed around the volume, in order to offer protection from external attack.
According to one embodiment, said volume is shaped so as to allow, in operation, a visibility in elevation of 10xc2x0 to 90xc2x0 and in azimuth of 360xc2x0.
According to one embodiment, the volume is spherical.
The subject of the invention is also a signal focussing device comprising a Luneberg lens having a homogeneous volume of dielectric, characterized in that the lens is produced by the process according to the invention.
According to one embodiment, said device is intended for the tracking of moving targets, especially nongeostationary satellites, for data exchange with at least one geostationary satellite, or for point-to-multipoint transmission, such as the Multipoint Multichannel Distribution System or MMDS.